Commissioning, clinical implementation, and performance of the Mobetron 2000 for intraoperative radiation therapy

Abstract The Mobetron is a mobile electron accelerator designed to deliver therapeutic radiation dose intraoperatively while diseased tissue is exposed. Experience with the Mobetron 1000 has been reported extensively. However, since the time of those publications a new model, the Mobetron 2000, has become commercially available. Experience commissioning this new model and 3 years of data from historical use are reported here. Descriptions of differences between the models are emphasized, both in physical form and in dosimetric characteristics. Results from commissioning measurements including output factors, air gap factors, percent depth doses (PDDs), and 2D dose profiles are reported. Output factors are found to have changed considerably in the new model, with factors as high as 1.7 being measured. An example lookup table of appropriate accessory/energy combinations for a given target dimension is presented, and the method used to generate it described. Results from 3 years of daily QA measurements are outlined. Finally, practical considerations garnered from 3 years of use are presented.


| INTRODUCTION
Intraoperative radiation therapy (IORT) aims to maximize the therapeutic ratio, which represents the balance between tumor control and normal tissue toxicity. At the time of tumor resection, it may be possible to provide a direct path between an accelerator and the tumor bed since the overlying tissues are moved out of the way. When the resection leaves behind a thin region of unresected tumor cells (either microscopic or macroscopic), the use of intraoperative high-dose-rate brachytherapy (IORT-HDR), low energy photons, or megavoltage electrons can provide a therapeutic dose to the tumor bed while minimizing damage to distal tissues. Accordingly, IORT has found several clinical applications using a number of different delivery devices for sites such as sarcomas, 1-3 breast, 4,5 recurrent head and neck cancer, 6 pancreatic cancer, 7 locally advanced and recurrent GYN tumors, 8 rectal cancers, 9,10 and genitourinary cancers. 11 Delivery of IORT is most easily performed in the operating theater (OR) within a sterile environment to avoid transferring the patient from the operating theater to a linear accelerator in a radiation oncology department. Several accelerators capable of being placed in an operating room have been marketed: • Mobetron, IntraOp Medical Corporation, Sunnyvale, CA (6,9,12 MeV electrons); and • NOVAC, Sordina IORT Technologies, Vicenza, Italy (4,6,8,10 MeV electrons). 13 To some extent, the commissioning of such a system is very similar to the commissioning of a normal therapeutic megavoltage electron linear accelerator or superficial unit. However, there are a number of important differences as well. 14 These differences include the lack of isocentricity and the use of dedicated applicators that are markedly different than conventional electron applicators.
This report details the implementation of an IORT program at our institution using the Mobetron, manufactured by Intraop Medical Corporation. This device presents a number of challenges for the medical physicist. These include the fact that many are used in unshielded rooms. The design of applicators for oblique treatments makes commissioning and dose planning challenging, particularly given the lack of a computer-aided planning system. We describe our approaches to these issues over the past 3 years.
Experience commissioning a previous model of the Mobetron, the Mobetron 1000, has been reported previously. 15 However, in the time since that publication a new model has become commercially available, the Mobetron 2000. In light of this, we focus on (a) characteristics that have changed in the new model (such as output factors), (b) supplemental measurements not included in previous publications, and (c) historical use data and experience at our institution. We devote minimal attention to items extensively covered elsewhere such as radiation protection shielding and TG-51 measurements. Overall, our goal is to provide the community with a broad set of beam parameters for comparison and to perhaps provide some guidance to help others develop their own programs using this modality.

| ME TH ODS
We begin by describing the Mobetron and its accessories, focusing on changes in the most current model. We then present our experience, dividing it into three categories: radiation protection, commissioning and dosimetry, and clinical procedures.

2.A | Mobetron description
The Mobetron is a dedicated mobile electron linear accelerator designed to deliver radiation during surgery. It uses X-band frequencies for electron acceleration in order to achieve smaller dimensions and a reduced weight more amenable to use in an operating suite.
The Mobetron 2000 is an improvement in this regard, with a total weight of approximately 3000 lbs. compared to roughly 4000 lbs. for the prior model. It is also a few inches shorter to accommodate lower ceiling heights. The Mobetron 2000 produces electrons with nominal energies of 6, 9, and 12 MeV at dose rates up to approximately 10 Gy per minute. The 4 MeV setting available in the previous model was eliminated to reduce QA load, instead bolus is used to achieve a similar effect. It is designed without a bending magnet to reduce leakage radiation so that it can be used in operating rooms with minimal shielding (e.g., designed for diagnostic imaging) or no shielding. It also incorporates a beam stop which attenuates approximately AE20°of the small quantity of bremstrahlung scatter created.
The accelerator and beam stop are mounted on a gantry with a limited rotational range of AE 45°. The accelerator itself can also tilt forward and backward (À10°/+30°), and translate in the vertical (30 cm), lateral (10 cm), and longitudinal (10 cm) directions.
There are 45 cylindrical anodized aluminum applicators, or cones, each 32 cm long. They range in diameter from 3.0 to 10.0 cm in 0.5 cm increments. For each diameter, there are three applicators with distal bevel angles of 0°, 15°, and 30°. The need for the beveled end is a reflection of the rotational capabilities of the Mobetron and the limitations on access to the tumor bed. The beveled applicators are for cases in which the tumor bed lies at some angle to the horizontal that is not within the Mobetron's range of motion and/or because of anatomical constraints with respect to the resection cavity. The beveled end of the applicator may be placed flush with the tumor bed while the applicator axis is at some angle to the normal axis. Two plastic boluses (0.5 or 1.0 cm thick) are available for each applicator that fit into the distal end to provide buildup and to reduce dose to critical structures that might lie beneath the tumor bed.
The Mobetron uses a soft docking system in which the accelerator is physically decoupled from the applicator which touches the patient (Fig. 1). The Mobetron 2000 includes an improved docking algorithm intended to reduce the time required to achieve docking.
A clamp is secured to the operating table and several pivoting arms hold a collar directly above the treatment site. The collar serves two purposes. The first is to securely hold the applicator itselfone end of which is fixed to the collar while the other rests on the tissue to be treated. Secondly, the other side of the collar (facing the linac) holds an annular mirror. A laser-detector scheme in the linac head is used to provide feedback regarding the absolute alignment (translation and rotation) of the linac and applicator. There is approximately a 4 cm gap between the collar and the end of the linac head.
For commissioning purposes, there is a spacer that attaches applicators directly to the head. This spacer places the applicator the correct distance from the source (maintaining the 4 cm air gap distance), and it obviates the need for a clamp and alignment procedure at each applicator change. A separate applicator with attached phantom is used for daily quality assurance measurements. It is intended to measure values of the depths of nominal d max and R 50 for each energy for output and energy verifications prior to treatment.
Other differences between the 2000 and 1000 include a solid state modulator included in the treatment unit. This eliminates the stand-alone modulator used by the 1000 which facilitates easier transportation and setup by reducing external cabling. The 2000 is powered by single-phase input, to minimize or eliminate the need for electrical modification of the OR suite. The treatment module itself rests on a powered jack for faster, easier transport. The control system is housed separately in a mobile cabinet which is placed outside the OR suite.

2.C | Dosimetric measurements
All dosimetric measurements were performed in a shielded linac vault in our department to permit the extensive beam-on time required for a comprehensive dosimetry characterization. Unless otherwise noted, all measurement were acquired in OmniPro Accept (IBA Dosimetry GmbH, Schwarzenbruck, Germany) with an IBA Electron Field Detector 3G and Reference Dosimetry Diode 3G in continuous ratio acquisition mode. Measurements were made in water with an IBA Blue Phantom and CU500E controller. Percent depth dose (PDD) and profile data were processed in OmniPro Accept.

2.C.1 | Output factors and air gap measurements
Output factors were measured for each applicator diameter, bevel, and energy combination. The rationale for the geometry of the measurements was described in the AAPM Task Group 48 report. 19 The applicators were positioned so that the beveled end was flush with the water surface. To position the diode, first a vertical depth-dose profile was measured, centered under the beveled end of the applicator. Then, an in-plane profile was measured at the depth of d max determined from the depth-dose profile. The diode was positioned at the location of d max of the in-plane profile and centered in the cross-plane for the output factor measurement. This was done to account for the fact that the beveled applicator axes were not normal to the water. Output factor measurements were normalized to the measurement for the 10-cm 0°applicator.
Air gap measurements were performed for each energy with 4, 7, and 10 cm diameter 0°bevel cones. Measurements were performed in water with an electron diode at the depth of d max . The applicator was initially placed flush with the water surface and subsequently retracted to measure with 0, 1, and 2 cm gaps. Air gap factors were produced by normalizing measurements to the 0 cm gap.

2.C.2 | Percent depth dose and lateral profiles
PDDs were acquired for every applicator, bevel and energy combination, normal to the water surface (i.e., not along the applicator axis for beveled applicators), and centered under the beveled end of the applicator. In-plane and cross-plane profiles were also acquired at a variety of depths for each combination of parameters. with and in contact with the solid water. Profiles were taken for each bevel angle and energy combination using 4, 7, and 10 cm diameter cones. All films were scanned on an Epson V750 flatbed scanner (Seiko Epson Corp., Tokyo, Japan) and converted to relative dose using a triple-channel dosimetry algorithm. [20][21][22] Each planar profile was normalized to d max to produce isodose distributions.
The extent of d 90 coverage in the in-plane direction and the central position of the coverage (for beveled applicators) relative to the applicator's center were tabulated for each measured 2D profile. This is illustrated in Fig. 2 To validate film measurements, PDDs from film were compared to PDDs measured with the electron diode and with a parallel plate chamber. In-plane profiles were also compared between film and the electron diode measurements.  F I G . 3. Applicator alignment example. The lookup tables assume that the front of the (beveled) applicator is just beyond the border of the target. This takes advantage of the characteristic shape of the 90% isodose coverage: the 'leading edge' of the coverage (i.e., the left hand side of the red line in the figure) exhibits a consistent lateral position with depth, whereas the 'trailing edge' varies considerably in its lateral position. The former conforms well to the hypothetical target, whereas the latter does not.

3.A | Radiation protection
Measurements of the instantaneous dose rate for the three energies were performed at 1 foot from all walls of the OR (one OR, two hallways, and the adjacent room housing the Mobetron control unit). It was also measured below the OR (a service driveway). Measured dose rates ranges from approximately 9 lSv per hour in the control area up to 90 lSv per hour in an adjacent hallway. The latter corresponds to 0.15 lSv per 1 Gy of delivered dose. Given the measured exposure rates, six or fewer patients per week can be treated in order to stay below regulatory limits (assuming an average patient dose of 20 Gy). This is about twice the permitted workload that would be determined using the table provided by Krechetov et al.,18 reflecting that the new design of the Mobetron was intended to reduce leakage radiation. In practice, two patients per week are the most that have been treated. QA measurements need to be scheduled with this in mind and occupancy in adjacent rooms may need to be controlled.

3.B.1 | Tg-51
Absolute calibration using the TG-51 protocol was performed in the same manner as with isocentric linacs using a calibrated ion chamber and electrometer. Accuracy of the calibration was confirmed by means of the Radiological Physics Center (currently IROC-Houston) service.     Selected profiles are plotted in Fig. 7.

3.B.4 | 2D Dose distributions
Profiles from film measurements were spot-checked against diode scans and ion chamber measurements (for PDDs) for validation purposes. Film was found to accurately reproduce these profiles, though artifacts were observed at shallow depths for some film profiles.
Comparison of film, diode, and ion chamber profiles are displayed in Lookup tables of applicator diameter energy combinations for 90% isodose coverage are presented in Tables 2-4. For superficial targets, a cone diameter slightly larger than the target (e.g., 0.5-1 cm) was sufficient. For targets with significant depth extent, the required cone size increased in order to offset isodose constriction.
This was exacerbated for targets requiring beveled applicators, as the applicator size must account for both isodose constriction and the fact that the cone axis is not normal to the surface.

3.C | IORT procedures
The results from 76 daily QA measurements (either pretreatment or as part of monthly QA) spanning 3 years are plotted in Fig. 13. reducing the likelihood that the observed apparent decline in output was permanent or significant. Table 5 and Fig. 14 provide a detailed breakdown of the treatment parameters used for 44 patients treated at our institution.
The majority of patients treated were sarcomas in the abdomen and pelvic regions. We have also treated several head and neck tumors.
One of the biggest issues during procedures was the type of operating     (Fig. 15). This support is being used routinely and has simplified and sped up the docking procedure markedly.
An important issue is the assignment of responsibilities for setting up the applicator and bolus sterilization and storage systems.

| CONCLUSION
Intraoperative radiation therapy with the Mobetron has become a standard procedure that the Department of Radiation Oncology provides to the hospital surgical services. Commissioning is a fairly lengthy procedure given the large number of applicators and lack of dedicated access to a properly shielded room. Integration of radiation oncology staff members and the Mobetron into the operating room setting is a challenge with a steep learning curve, but a cooperative, proactive approach can greatly ameliorate these issues.

ACKNOWLED GMENTS
We thank the radiation oncology and operating room staff at the Mayo Clinic, Scottsdale, AZ; particularly Gary Ezell, Ph.D. for graciously allowing us to observe their procedures and for their invaluable advice. We also thank Don Goer, Shura Kretchetov, and Tom Cook of IntraOp Medical Corp. for helpful discussions and advice.
We would also like to acknowledge the cooperation, support, and patience of the surgeons, nurses and OR staff members at UWMC in helping us implement this program successfully.